1. Field of the Invention
The present invention relates to the field of designing radio receivers using digital signal processing techniques.
2. Prior Art
The following references are relevant to the present invention:
[1] F. de Jager, "Delta modulation--a method of PCM transmission using the one unit code," Philips Res. PA1 [2] H. S. McDonald, "Pulse code modulation and differential pulse code modulation encoders," 1970 U.S. Pat. No. 3,526,855 (filed 1968). PA1 [3] R. Steele, Delta Modulation Systems, New York; Wiley, 1975. PA1 [4] H. Inose, Y. Yasude, and J. Murakami, "A telemetering system code modulation--.DELTA.--.SIGMA. modulation, " IRE Trans. Space Elect. Telemetry, vol. SET-8, pp. 204-209, September 1962. PA1 [5] S. K. Tewksbury, and R. W. Hallock, "Oversampled, Linear Predictive and Noise-Shaping coders of order N&gt;1, " IEEE Trans. Circuits Sys., vol. CAS-25, pp. 436-447, July 1978. PA1 [6] D. B Ribner, "Multistage bandpass delta sigma modulators," IEEE Trans. Circuits Sys., vol. 41, no. 6, pp. 402-405, June 1994. PA1 [7] A. M. Thurston, "Sigma delta IF A-D converters for digital radios," GEC Journal of Research Incorporating Marconi Review and Plessey Research Review, vol. 12, no. 2, pp. 76-85, 1995. PA1 [8] N. van Bavel et al., "An analog/digital interface for cellular telephony," IEEE Custom Integrated Circuits Conference, pp. 16.5.1-16.5.4, 1994.
Repts., vol. 7, pp. 442-466; 1952.
There are many advantages in using digital signal processing (DSP) techniques in the implementation of radio frequency (RF) receivers. Harnessing these advantages, however, relies to a great degree on the ability to effectively convert the signal from the analog to the digital domain.
In conventional RF receiver implementations, the received signal is down converted to in-phase (I) and quadrature (Q) baseband components via one or more conversions to an intermediate frequency (IF), using analog circuitry, and then converted to the digital domain using a pair of pulse coded modulator (PCM) type analog to digital A/D converters operating at baseband. A number of sources of degradation exist in using this design approach that limit the achievable performance. Any phase error in the local oscillators used to mix the signal to I and Q baseband components will impair the receiver's ability to discriminate between signal components above and below the IF center frequency. For example, achieving 40 dB of (I-Q) discrimination requires these local oscillators to be orthogonal to within 0.5.degree., including all drift from aging, temperature and manufacturing tolerances. This phase accuracy must then be maintained throughout the pair of analog paths up to and including the A/D conversion function. Similarly, the amplitude response of the two analog paths, including any gain mismatch between the two A/D converters, must be well matched to preserve the (I-Q) discrimination of the receiver. Again, to obtain discrimination of 40 dB, it is necessary to match the amplitude response of the two paths to better than 0.1 dB. Such tolerances are possible and may be exceeded by using a calibration routine; however, obtaining this tolerance in a pair of digital paths is routine and therefore provides motivation of digitizing an IF signal directly and thereby avoiding these balancing issues altogether.
Design approaches for direct A/D conversion of the received IF signal using conventional PCM type multiple bit A/D converters eliminate the need for the IF/Baseband analog circuitry. Although the location of a substantial number of high-speed digital switches alongside sensitive RF circuitry invites interference, the potential benefits are often considered to outweigh the new design difficulties. Another problem introduced by the digital processing of IF signals is the need to perform high-speed A/D conversion, a problem compounded by the need for higher linearity in early stages of the receiver. Conventional multiple bit A/D converters have the property that the signal bandwidth available is equal to one half of the sampling frequency, less a margin to allow for anti-alias filtering. The product of the bandwidth and resolution of a converter (or dynamic range) is a measure of its performance, and this will typically be reflected in the difficulty of designing the device and also in its market price. Because a typical IF signal is narrowband compared to its carrier frequency, the use of wideband multiple bit converters does not represent an optimal coding solution to a very specific problem. Some reduction in the A/D converter's processing overhead can be achieved by operating it in a subsampled mode such that the carrier frequency is above the sampling frequency. However, achieving the bandwidth and dynamic range design goals with this method requires enhanced channel filtering prior to the conversion to prevent other channels from aliasing into the passband resulting in an increase in cost and power consumption.
A/D converters designed based on the principles of predictive and interpolative coding (such as delta converters and sigma delta converters), although traditionally operating on baseband signals--especially audio--exhibit attractive properties (see the foregoing references). First, they are an over-sampled coding technique that achieves coding accuracy by fine temporal quantization rather than fine level quantization. Thus, for a given sampling frequency, the usable bandwidth is very much reduced compared with standard pulse code modulation (PCM) techniques, and this trade-off in requirements is reflected by a simplified design suited to low tolerance components. In general, the analog filtering required with such a converter is thus comparatively simple.
A second advantage of these types of coding is their inherent linearity. A multiple bit converter is very susceptible to component tolerances, and a non-linear mapping between the analog and digital domains is difficult to avoid. One very successful means of combating this effect is by the use of high-level additive dither, which effectively decorrelates the non-linearities from the input signal and reduces the effect to a benign noise source. This technique may be used to remove the non-linear effects from the coder, but the limiting performance is ultimately that of a PCM code, and this itself can introduce highly correlated distortion, which in an application comprising evenly spaced radio channels is likely to present difficulties.
The use of interpolative type encoders (i.e., sigma delta converters) in the analog to digital conversion of a high frequency IF have been advocated by many authors, such as the authors of the last two references hereinbefore set forth. Although the advantages of these techniques are clearly delineated by these authors, there remain numerous implementation challenges which must be overcome by a designer who is focused on achieving the low cost and low power consumption goals. The most relevant of these challenges is the fact that although these techniques ultimately produce an oversampled single bit (1-bit) digital representation of the IF signal, the signal must first be converted from its analog continuous-time representation to an analog discrete-time-representation, where it is processed by elaborate discrete-time analog circuitry prior to being mapped into the digital domain (i.e., quantized or digitized). Furthermore, achieving the high dynamic range and the low quantization noise advantages offered by these techniques often requires the implementation of high order encoding loops, with considerable increase in complexity.